(1) Field of the Invention
The invention is related to an optical liquid level sensor.
(2) Description of Related Art
Precise sensing of the level of liquids in tanks or containers is important for different reasons: prevention of overfilling of tanks, computation of the actual liquid consumption, identification of low liquid level, etc. From a safety point of view, it is even more important to know the exact level for tanks with inflammable liquids such as fuel. It is therefore necessary to control the filling and consuming processes as well as determining the current level if filling or consuming is to be started. Especially with inflammable liquids it is furthermore important for safety reasons to reduce or to eliminate all kind of electricity from inside the tank or container. It is important to avoid any kind of spark or ignition source inside the area/space of the liquid. Furthermore, in cases of liquid level sensors in a tank/container for aircrafts or vehicles it is important to design the liquid level sensor in such a way, that the components satisfy the crash requirements and that the liquid level sensor in case of a crash does not cause any damage to the tank/container, even if the tank/container is made of soft material.
The document U.S. Pat. No. 7,710,567 describes determining the density and/or temperature of a fluid in a tank, based on the optical energy affected as it propagates across a gap between opposing end faces of optical waveguides. A housing includes a cylindrical body, a top portion and a bottom portion. The top and bottom portions are secured to the body by fasteners. A conduit is positioned in a volume within the housing. The top and bottom portions each have a through hole formed therein. The through holes are aligned with a passage defined by the conduit. The through holes permit the fluid in the tank to enter. Legs are attached to the bottom portion to a bottom surface of the tank. A plurality of sources of optical energy are in optical communication with an associated one fiber optic cables, e.g. light-emitting diodes having emission in the visible green wavelength. Also, each light detector is e.g. a photo detector sensitive in the visible green wavelength.
The document US 2004/0021100 describes fiber optic measuring of levels of fluid. An ordered array of multiple optical fibers contains each a single sensitive element located on a specific level within the range of fluid level change. Each single sensitive element transmits different light signal, depending on either the sensitive element is immersed or located above the level of liquid. The making of optical contact of the fiber is with cladding disposed from the tank bottom with the fiber without cladding disposed from the top at a certain level of the range of liquid levels By removing a section of cladding near the end of the fiber the latter is equipped with a fluorescent element at its end. The fibers are connected to a common reflective mirror or a fluorescent element at the bottom of a housing. A bundle of optical fibers is disposed along a holder with the sensitive sections distributed along the holder. In side cross-sectional view, the U-shaped sensitive sections are of different types formed by e.g. a fiber without cladding or isolation, however with metallic cover or cladding at the lower part of U-shape or loop. Receiving parts of the fibers form a feedback bundle of fibers guiding the light signals to the light detector. The light signals modulated in frequency domain are detected by a single light detector which transforms them in current/voltage signals.
The document U.S. Pat. No. 3,844,171 describes a liquid level indicator having a probe which is inserted within a container holding the liquid, and an external readout device. A unit includes a connector end which holds coupling lines by means of a clamp, into a middle section which holds a sensing device axially extended therefrom. The sensing device is a hollow tube from glass or plastic with the middle section having one end thereof knurled for manipulating and the other end with a screw thread to connect the unit to a holder, permanently attached to the top of the container. The probe contains a vertical prism and pairs of light guides spaced at preselected levels along the prism. The light guides are spaced from the prism. In an elevation sectioned view of the unit, the sensing device is mounted on a retaining member located adjacent a notch longitudinally cutting a section of the wall of the tube. Part of the middle section is having an external thread thereon to permit attaching the unit on the container for permanent connection. The tube needs not be transparent. The fiber optics continues from the upper section in a semicircular arrangement around the tube.
The document WO2010051806 discloses an optical sensor arrangement for detecting a first liquid medium in a second liquid medium by means of reflection of an emitted light beam at a wavelength, with a light source and an associated receiver, wherein two round glass rod lenses which are encapsulated in a housing are arranged parallel to one another. The glass rod lenses have an optical refractive index which is different from that of the liquid media. A reflection surface which is connected to the housing is arranged opposite the glass rod lenses. There is also a control device having a beam splitter, a second receiver and a third receiver which are arranged opposite one another. This sensor is suited only for measuring whether the liquid has reached a certain predefined level or not, but it is not suited for continuous measurements of varying levels. This sensor is therefore especially useful as a low level or maximum level sensor, but it cannot provide information e.g. for computing the consumption.
The document U.S. Pat. No. 4,134,022 discloses a level sensing apparatus having a source for supplying a signal having a predetermined frequency, a level sensor connected to the source and having an output for supplying an output signal having the predetermined frequency as long as the material, the level of which is being sensed, is not at a predetermined level. The level sensing apparatus further has a frequency sensitive circuit for receiving the output signal from the level sensor and for providing an output whenever the frequency of the signal is above or below the predetermined frequency, and a load which is connected to be responsive to the output from the frequency sensitive circuit. The level sensing apparatus is only capable to determine whether the liquid has reached a predefined level or not. Even the extension of the basic concept to several sensors is limited in its use, because it can only determine whether the liquid in one of the tanks has reached the respectively defined level or not. It is not possible to determine the exact level of the liquid.
The document U.S. Pat. No. 3,535,933 discloses a liquid level indicator in which a plurality of depending parallel light transmitting rods having different lengths are adjacently positioned, but maintained physically separated substantially throughout the entire longitudinal extent thereof. The upper portions of each of the light rods are interconnected and spaced apart by a flange section which in turn is clamped to the liquid container. The lower portions of the rods are integrally connected by a separator element between the rods and the upper portions provided an exposed liquid level indicating surface. The lower extremities of the light rods are positioned in the liquid and define a conical tip having an included angle of 90° for receiving light rays from and reflecting light rays to the upper indicating surface. The liquid level indicator needs a plurality of elongated members or rods and each of them needs a respective sensing unit. In order to provide the required stability, these members or rods need to be massive and have therefore significant weight. In addition, the light source and the light measuring device need to be positioned on the same end of the members or rods.
The document EP1533598 discloses a fiber optic level measurement device for use with transparent liquids in containers. Said fiber optic level measurement device has at least one optical fiber that is at least partially immersed in the liquid with the liquid level determined from the difference between injected light intensity and exiting light intensity. An input leg of the optical fiber has a spiral shape which extends to the lowest level of the container, while the output leg of the fiber is straight. Only one fiber measures the level of liquid based on the amount of refraction in the liquid. Therefore the measurement and the comparison of the light intensities of the input light and the output light needs to be very precise in order to determine the liquid level. Respective devices for measuring the light intensity need to be calibrated very precisely and are fairly expensive. Furthermore, the resolution of the sensor is limited due to the measuring principle.
The document DE3235591 discloses a fiber-optic liquid-level display device with a first optical fiber, assigned to a light source, and a second optical fiber, assigned to a light detector. The two optical fibers are coupled via a fiber coupler to a third optical fiber and a fourth optical fiber. The third optical fiber and the fourth optical fiber reach with their ends to different depths into the vessel containing the liquid to be measured. Depending on the level, the light at the ends of the optical fibers is coupled out or reflected and fed to the photodiode, with the result that the level range of the liquid can be detected by means of the current of said photodiode. The teaching of DE3235591 allows a reduction of the number of components needed, but it increases the requirements for the photo diode significantly, since it needs to discriminate the exact light intensity. The more different levels are to be detected, the more difficult is the realization of this sensor. This is therefore limited with respect to the achievable resolution.
Capacitive sensors as proposed in documents DE102008064019, DE102008042254 or DE102009029099 suffer from the disadvantage that electricity has to be transmitted into the container/tank which is not desirable, especially for sensing the level of inflammable or explosive liquids. In addition, most capacitive probes contain metal probes and therefore tend to be in conflict with the crash requirements.
For preparing the application, the following prior art documents were considered: DE102008064019 KOSSIRA 2008; DE102008042254 ROESSLER 2008; DE102009029099 STAMMLER 2009; DE3235591 SPENNER 1982; DE4204212 WITTKOWSKI 1992; EP0152644 MULDER 1983; EP1533598 SCHNELL 2003; U.S. Pat. No. 3,535,933 PLIML 1969; U.S. Pat. No. 3,844,171 RODGER 1971; U.S. Pat. No. 4,134,022 JACOBSEN 1976; U.S. Pat. No. 4,373,389 DECKER 1979; U.S. Pat. No. 5,802,728 KARNICK 1995; U.S. Pat. No. 7,710,567 MENTZER 2006; US 2004/021100 GOUZMAN 2002 and WO2010051806 WILDSHUTZ 2008.